class Program
{
    const string CompanyId = "*******";
    const string OAuthClientId = @"*****";
    const string OAuthClientSecret = @"******";
    const string OAuthAccessToken = @"*******";
    const string OAuthRefreshToken = @"*******";
    const string OAuthTokenEndpoint = @"*******";
    const string MinorVersion = "**";

    static string connectionString = $"Url=https://sandbox-quickbooks.api.intuit.com;Company Id={CompanyId};Use SandBox=true;OAuth Client Secret={OAuthClientSecret};OAuth Client Id={OAuthClientId};" +
                    $"OAuth Access Token={OAuthAccessToken};OAuth Refresh Token={OAuthRefreshToken}; OAuth Token Endpoint={OAuthTokenEndpoint};Minor Version={MinorVersion}";

    static void Main(string[] args)
    {
        ReadOperation();
    }

    static void ReadOperation()
    {
        using (var conn = new C1QuickBooksOnlineConnection(connectionString))
        {
            conn.Open();

            DataTable databaseTables = conn.GetSchema("Tables");
            Console.WriteLine("List of Tables in database: \n");
            foreach (DataRow row in databaseTables.Rows)
            {
                //Display table names
                string tableName = row["TableName"].ToString();
                Console.WriteLine(tableName);//Table Name

                //Get column names in a table
                DataTable dtColumnsSchema = conn.GetSchema("Columns", new string[] { tableName });

                //Remove column DbType
                dtColumnsSchema.Columns.Remove("DbType");

                //Add column References
                var fkCol = dtColumnsSchema.Columns.Add();
                fkCol.ColumnName = "References";

                //Populate data for column References
                DataTable dtForignKeys = conn.GetSchema("foreignkeys", new string[] { tableName });

                foreach (DataRow fkRow in dtForignKeys.Rows)
                {
                    //Name of the column that is a foreign key
                    var fkColName = fkRow["ColumnName"];

                    //Find corresponding data row in dtColumnsSchema
                    var dtRow = dtColumnsSchema.Rows.Cast<DataRow>().Where(r => r["ColumnName"].ToString().Equals(fkColName)).Select(r => r).First();

                    //Fill value
                    dtRow["References"] = $"{fkRow["ForeignKeyTableName"]}.{fkRow["ForeignKeyColumnName"]}";
                }
                dtColumnsSchema.ToCSV($@"E:\temp\QuickBooksOnline\{tableName}.csv");
            }
        }
    }
}

static class CsvHelper
{
    public static void ToCSV(this DataTable dtDataTable, string strFilePath)
    {
        using (StreamWriter sw = new StreamWriter(strFilePath, false))
        {
            //headers   
            for (int i = 0; i < dtDataTable.Columns.Count; i++)
            {
                sw.Write(dtDataTable.Columns[i]);
                if (i < dtDataTable.Columns.Count - 1)
                {
                    sw.Write(",");
                }
            }
            sw.Write(sw.NewLine);

            foreach (DataRow dr in dtDataTable.Rows)
            {
                for (int i = 0; i < dtDataTable.Columns.Count; i++)
                {
                    if (!Convert.IsDBNull(dr[i]))
                    {
                        string value = dr[i].ToString();
                        if (value.Contains(','))
                        {
                            value = String.Format("\"{0}\"", value);
                            sw.Write(value);
                        }
                        else
                        {
                            sw.Write(dr[i].ToString());
                        }
                    }
                    if (i < dtDataTable.Columns.Count - 1)
                    {
                        sw.Write(",");
                    }
                }
                sw.Write(sw.NewLine);
            }
            sw.Close();
        }
    }
}